The present invention relates to a rotary developing device comprising a cylindrical rotary developing unit, a plurality of development members mounted around the periphery of said rotary developing unit, and a driving means, wherein the rotary developing unit is driven by the driving means to bring one of the development members to the developing position facing a photoreceptor and subsequently the transmission of the driving power to the development member is conducted
In a conventional multi-color image forming apparatus employing a rotary development method, a plurality of development members are mounted along the periphery of a rotary developing unit and the rotary developing unit is driven to sequentially bring the development members to a developing position so as to carry out the development operation. For this, a driving means for rotating the rotary developing unit and a driving means for rotating a development roller or the like built in each development member mounted on the rotary developing unit are separately provided.
The rotary developing unit with the aforementioned plurality of development members is generally cylindrical and is provided around its outer periphery with heavy parts such as development rollers as developer carriers which are metallic rollers or metallic shafts covered by elastic material for developing latent images formed on a latent image carrier such as a photoreceptor. Accordingly, the rotary developing unit has large moment of inertia.
In case of a rotary developing unit having four color development members as a general example for multi-color printing, development is carried out by driving the rotary developing unit to rotate 90 degrees four times so as to sequentially bring the four color development members to a position facing the photoreceptor. As arrangements for retaining the rotary developing unit in the state after the rotary developing unit is stopped at the developing position where the development operation is conducted, there are a case of using the retaining force of a motor itself and a case of providing an engaging member separately.
As the moment of inertia is large for conducting 90-degree rotation of the rotary developing unit, the motor as the driving means must produce a large force in proportion as the moment of inertia. For raising the printing speed, the increase in speed of conducting the 90-degree rotation is effective. However, as the speed of the 90-degree rotation is increased, the acceleration during the rotation is increased. Since the power required for the driving means should be a square of the acceleration relative to the moment of inertia, the required power must be great.
The required power for rotating the rotary developing unit produces in turn the contrary effect on stopping the rotary developing unit. For stopping the rotary developing unit, the driving means carries out a braking function to reduce the rotational force of the rotary developing unit. Ideally, the rotational force of the rotary developing unit is reduced to zero by the braking force just before the rotary developing unit is stopped.
In the electrophotographic technology, a stepping motor is generally used for driving the rotary developing unit because it can achieve the short-time acceleration and achieve the higher positioning accuracy with relatively simple control and a DC brushless motor is generally used for driving the development members because it can provide high torque efficiency and it never breaks down due to load fluctuation.
When a DC brushless motor, not a stepping motor, is used for driving the rotary developing unit, an encoder is fixed to the output shaft of the motor because the DC brushless motor has poor positioning accuracy. The rotation angle of the motor is determined from signals from the encoder. According to this information, the rotation angle of the rotary developing unit is controlled. In addition, the DC brush less motor has poor acceleration. Accordingly, since a time lag is generated in transmitting and receiving of the signals from the encoder, it is impossible to rotate the rotary developing unit at high speed.
The feedback control according to the signals from the encoder increases the load on a CPU as a controller. The increase in load on the CPU restricts the entire operation of the apparatus. From the above reasons, the use of a stepping motor is common for driving the rotary developing unit. However, even stepping motor has problems such as vibration, noise, and smaller flexibility for load fluctuation. Because of the smaller flexibility, the motor should break down to stop the operation at its worst. In addition, stepping motor is expensive as compared to other motors such as DC brushless motor relative to the same torque.
On the other hand, nowadays there are some types employing a stepping motor, not a DC brushless motor, for driving the development members. One of triggers for the employment of stepping motor is that decrease in cost of stepping motors has been achieved as compared to other motors. Moreover, its shorter acceleration/deceleration time has come to the fore as a merit. For achieving the increase in printing speed, it is desired to shorten the acceleration time for raising the speed of a development roller to the steady state velocity as well as the time for switching colors according to the rotation of the rotary developing unit.
For actually switching between the rotary unit driving action and the development member driving action, the rotary developing unit is driven to rotate 90 degrees four times so as to sequentially bring development cartridges to a position facing the photoreceptor and a development input gear of the development cartridge, brought to the aforementioned position, is meshed with a development member driving gear, whereby the transmission of the driving power to the development roller is conducted. During this, the pitch circles of the gears meet so that their tooth tops may collide with each other in some cases. In the event of collision, the driving means may develop trouble (may stop due to breakdown of the motor) and may produce image defects due to vibration generated by the collision.
Due to backlash and deflection existing in a driving-side gear train and deformation of the rotary developing unit itself, rotational force may remain. The remaining rotational force is transmitted as vibration to the entire device through the driving means when the rotary developing unit is stopped. The vibration may be transmitted to an exposure means or a latent image carrier. In this case, the vibration produces registration error during formation of latent image. The vibration may be transmitted to a transferring section. In this case, the vibration produces transferring error.
Further, when a driving means for rotating the development roller starts to operate just after the rotary developing unit is stopped, uneven rotation of the driving means or vibration generated in the driving means is transmitted to the entire device. Similar to the vibration generated at the stop of the rotary developing unit, the vibration after the stop of the rotary developing unit produces image defects such as registration error.
If the aforementioned two driving means are of different kinds or having lots of different sizes, the driving means have different rotation and vibration characteristics. Even when the driving means have substantially the same vibration characteristics, since these are disposed in different places, these are influenced by the characteristics of the respective places when subjected to vibration. Vibrations generated by different vibration sources may be composed of different components. These vibrations may not damp each other and may be sometimes superposed to by synthesized i.e. amplified, thus producing image defects such as errors in longer period.
When a stepping motor is used for driving the rotary developing unit and a DC brushless motor is used for driving the development roller, the stepping motor is in the stopped state during the action of driving the development roller (this action will be sometimes referred to as “the development member driving action”). The stopped state means that the motor is energized with a minute electric current to retain the rotor at a predetermined position. In case of having an external locking mechanism for retaining the rotary developing unit at a predetermined position, it is possible to cancel the retention of the rotor. However, once the retention of the rotor is cancelled, the position of the rotor should be unstable. In this case, the position of the first exciting phase for the next action for driving the development roller is not certain, causing position error and thus reducing the rotational accuracy of the rotary developing unit. Consequently, the retention of the rotor is indispensable to maintain the rotational accuracy of the rotary developing unit.
However, the retention of the rotor requires the consumption of electric power. Since the motor is energized but the motor itself does not rotate, the energy applied to the motor becomes heat energy, increasing the temperature. The increase in temperature leads to drop in torque.
When stepping motors are used for driving the rotary developing unit and for driving the development roller, respectively, drivers for controlling the stepping motors and timers for controlling the drivers are required, respectively. Besides the aforesaid timers, another timer of a longer cycle is also required for retaining the stepping motor for driving the rotary developing unit in the stopped state during the development member driving action.
In the rotary developing unit, the amounts of developers in the respective developer cartridges vary according to the development operation so that the load balance of the cylinder varies delicately. The variation in load balance increases the moment of inertia in the rotary developing unit, thereby increasing the torque required to the stepping motor. Accordingly, it is required to periodically match the motor torque in a very short cycle during the action of driving the rotary developing unit (this action will be sometimes referred to as “the rotary unit driving action”). This is because the timer of a very short cycle is used. If the timer of a very short cycle is used also for retaining the stopped state, the CPU should be overdriven and thus restrict the other operation. Addition of such drivers and timers makes the substrate structure complex and also makes the control, including the control of the CPU, complex. It should be understood that the addition of such drivers and timers increases superposed driving time, leading to increase in electrical consumption.